Apparatus for detecting and analyzing low gaseous concentrations



NOV. 12, 1963 LOVELQCK 3,110,809

APPARATUS FOR DETECTING AND ANALYZING LOW GASEOUS CONCENTRATIONS Filed Sept. 3, 1959 INVENTOR JAMES EPHRAIM LOVELOCK United States Patent 3,110,809 APPARATUS FOR DETECTING AND ANALYZING LOW GASEOUS CGNCENTRATIONS James Ephraim Loveloclr, London, England, assignor to National Research Development Corporation, London, England, a British corporation Filed Sept. 3, 1959, Ser. No. 837,924 Claims priority, application Great Britain Sept. 12, 1958 7 Claims. (Cl. 25083.6)

This invention concerns improvements in or relating to the detection and/or analysis of low concentrations of gases and vapours. It is an improvement in or modification of the invention described and claimed in a co-pending application filed in the United States of America under application No. 733,429 which will hereinafter be referred to as the earlier application.

In the earlier application there is disclosed inter alia a method of detecting and/ or measuring low concentrations of gaseous or vapourized substances in which the said substances are introduced in low concentration into an atmosphere of a rare gas having an excitation potential equal to or higher than the ionization potential of the substances, and substantially free from impurities of lower excitation potential, the rare gas atoms being excited partially to a metastable state, and in which the concentration of ions in the presence of said substances is determined relative to the concentration in the absence thereof.

In the earlier application there is also described and claimed apparatus for use in carrying out the above method comprising a detector chamber adapted for connection to a source of rare gas, means for rendering the rare gas atoms partially metastable within the chamber, and means for determining the concentration of ions in the rare gas atmosphere in the presence or absence of low concentrations of gaseous or vapourized substances. This apparatus is intended for use in conjunction with a gas chromatography column, which as described is constituted by a glass tube approximately 4 feet in length and 0.4 centimeter in diameter, surrounded by a casing enclosing suitable insulating materials and heating elements, and packed internally with a particulate adsorbent material, i.e. a powder such as a diatomaceous earth, which is impregnated with a substantially involatile high boiling point hydrocarbon oil fraction acting as the stationary phase. Using such gas chromatography columns, a flow rate of argon (or other rare gas) of from 10 to 100 millilitres per minute is customary.

A recent development, however, has occurred in the art of gas chromatography, namely the use of chromatography columns made from narrow-gauge metal capillary tubing. In this new technique, instead of the columns being filled with a powder coated with the stationary phase (i.e. the involatile hydro-carbon oil fraction) the latter is merely spread in a thin film over the inner surface of the capillary tube. It is possible with such capillary tubes to construct columns which, while occupying only a small and convenient amount of space, are several hundred feet long. Compared with the packed powder columns hitherto in use this new type of capillary column has a greatly enhanced performance, both in terms of separation efiiciency and rapidity of analysis. The small quantity of stationary phase spread over the inner surface of the capillary tube can, however, handle only a very small quantity of the mixture of gases and/ or vapours to be separated, usually not more than a few micrograms. The optimum rate of gas flow through these capillary columns is, moreover, small as compared with that through conventional packed powder columns, being only of the order of one millilitre per minute. For these reasons, and also because of the great separation elficiency of the capillary columns, the separated fractions are carice ried by only a few microlitres of carrier gas, and even at their very small flow rates such minimal quantities of carrier gas may emerge in a time of 1 second or less. Any detector for use with these capillary columns must therefore be very highly sensitive if it is to succeed in resolving the components of the mixture.

The method of detection. described and claimed in the earlier application is outstandingly sensitive, and in principle it is well able to deal satisfactorily with these extremely small quantities. In view of the low flow rate of the gas stream, and the high localization of the fractions within it, however, it will be clear that the detector chamber must itself possess only a correspondingly very small volume if satisfactory resolution is to be achieved. The actual detector chamber described and shown in FIG- URES 2 and 3 of the earlier application in conjunction with a packed powder chromatography column, possesses a volume of the order of 5 millilitres, which is far too great for satisfactory resolution of the output of a capillary column.

Nor is it possible merely to reduce the dimensions of the detector chamber previously described. For an understanding of this fact it is necessary to restate certain principles governing the mode of operation of this detector. Detection depends upon measurement of the ionization current (or some related factor) in the detector chamber which increases markedly when certain gaseous or vapourized substances are introduced into the argon or other rare carrier gas stream. This increase is due to the molecules of these substances being ionized by collision with metastable rare gas atoms, which in turn have been established in the carrier gas by collision between unexcited rare gas atoms and free electrons, liberated initially in small quantity by bombardment of the gas atoms from a source of ionizing radiation and accelerated to the requisite energy level by an applied electric field, and multiplied by release of further or secondary electrons when metastable atoms collide with other molecules. A high concentration of metastable rare gas atoms in the gas is obviously desirable if the detector is to have maximum sensitivity, but to achieve this theremust be not only an electric field with a potential gradient sufiiciently great to accelerate free electrons to an energy level able to excite rare gas atoms with which they collide into the metastable state, but there must also be an adequtae initial liberation of primary electrons by the ionizing radiation. The intensity of this is, however, limited by radiation hazards, and hence the provision of an adequate supply of free primary electrons necessitates that the ionizing radiation should be directed into a substantial volume of gas, say from 1 to 10 millilitres. It is for this reason that mere reduction of the dimensions of the detector chamber described in the earlier application will not alone sufiice to render it useful in connection with the latest capillary chromatography columns.

It is the purpose of this invention to provide a modified arrangement for detection with an effective volume sufiiciently small for use with capillary chromatography columns.

The invention is based upon the realization that, if the ion collecting electrode is made small compared with the dimensions of the detector chamber, the steepest part of the potential gradient will then reside in the vicinity of the collecting electrode, so that free electrons Will be accelerated to an energy level suflioient to excite the rare gas atoms into the metastable state only near the surface of the electrode. By this means the effective volume of the detector chamber where sensing occurs can be restricted to quite a small space immediately surrounding the collecting electrode, whilst the volume of gas available for the production of electrons by irradiation remains as large as before. The construction of a practical small-volume detector on these principles then requires only that the gas from the chromatography column shall be led directly into the sensing volume, and also that some means be provided for preventing the detection of vapour molecules ionized outside the sensing volume by collision with metastable atoms always present in small concentration in the larger volume of the rest of the detector chamber.

According to the present invention there is provided a method of detecting and/ or measuring low concentrations of gaseous or vapourized substances according to the earlier application, for use with a capillary chromatography column, in which the detector chamber in which ionizing radiation may be directed to liberate primary electrons possesses a substantial volume (which may be of the order of l to millilitres), the sensing volume within the chamber is restricted in operation to a space immediately adjacent both the collecting electrode and the discharge point of the column by the use of a collectin electrode which is very small in relation to the volume of the chamber and is closely adjacent the said discharge point and a scavenging gas flows through the chamber to sweep out of the chamber mixture escaping from the sensing volume. By using an electrode which is very small in relation to the dimensions of the chamber a sharp potential gradient is established in the immediate vicinity of the electrode. A point electrode or a bead electrode with dimensions very much less than those described and shown in the earlier application are both suitable whereas the overall dimensions of the detector chamber may be approximately the same as in the earlier application.

Acconding to one feature of the present invention the end of the capillary chromatography column is used as the collecting electrode, both the inner and outer diameters of the tube being rounded so that the edge of the end is an annulus of approximate semi-circular section. A preferred form of electrode is, however, constituted "by mounting at the end of the capillary column a short length of gold wire whose outer end has been fused to form a small spherical bead.

The scavenging gas may be pure argon or it may be another gas, for example air or nitrogen, able to discharge metastable rare gas atoms without production of ions. The sensing volume may be shielded from the flow of scavenging gas.

Shielding the sensing volume from the flow of scavenging gas may, according to the invention, be carried out in various Ways. In a preferred arrangement the electrode and the sensing volume are within a hollow cylinder open at one end to the column and at the other end to the chamber. In another arrangement the mixture emerging from the column is made to seep through a porous metal plug which forms the electrode, whereby the sensing volume is restricted to the film of gas immediately adjacent the surface of the plug.

Apparatus according to the invention may comprise a metal chamber open at one end, an insulating plug closing the open end of the chamber, and a hollow insulating member enclosing the end of the capillary column projecting through the insulating plug into the chamber.

The capillary chromatography column may be constituted by a metal tube and the end thereof inside the chamber may constitute the electrode, the inner and outer diameters of the tube end being rounded as aforesaid. Alternately the electrode may be a bead formed at the end of a short length of small diameter wire attached to the end of the column.

Admission and exit passages for scavenging gas may be formed in the closed end of the chamber and through the insulating plug respectively, and shielding means may be provided to shield the sensing volume from the flow of scavenging gas.

In order that the invention may be well understood one embodiment and a modification thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIGURE 1 is a sectional elevation of a [detecting chamber according to the invention, drawn to a greatly enlarged scale; and

FIGURE 2 is a modification thereof.

Referring to FIGURE 1 there is provided a housing 11 which is conveniently made of metal, for example, brass, and is preferably cylindrical in shape. The housing is bored on the axis of the cylinder from one end to form a chamber 12 which is closed by an insulating plug 13. The latter may conveniently be made of polytetrafluoroethylene. At its opposite end the housing has :an inlet passage 14 from which a stream of scavenging gas, for example, argon, may be introduced and the insulating plug 13' is provided with one or more outlet passages 15 for the said scavenging gas.

The end 16 of a metal capillary tube which constitutes the chromatography column passes through an insulating member 17 which projects through the insulating plug 13 part-way into the chamber. The member 17 forms an extension of the plug 13. The capillary tube may, for example, be of 0.01 inch internal diameter and of any desired length, and both the inner and outer diameters at the end in the chamber are rounded. The inner end of the insulating member 17 is bored out beyond the end of the capillary tube 16 to form a small hollow jacketing cylinder 18. A short length of small diameter gold wire 19 has its end fused to form a head 20. The wire 19 is crimped or otherwise formed so that it may be pushed into the bore of the capillary tube 16 and is held in a desired position by friction, while allowing clearance for the stream of gas from the capillary tube to pass it and flow into the chamber 18. The wire 19 is inserted into the capillary tube to such a depth that the bead 2% is approximately in the centre of the cylinder 18, which latter encloses the sensing volume around the bead or electrode 20 and shields it from the flow of scavenging gas. A suitable source of radiation 21 which may consist, for example, of a radium or radium-D source of alpha-particles, or a strontium source of betaparticles, is disposed around the inner diameter of the chamber 12. Alternatively, the recently introduced source of beta-particles containing tritium may be used.

The metal capillary tube 16 is connected to suitable circuitry for detecting changes in ionization current. Such circuitry is illustrated purely to facilitate description and may comprise a source of potential represented by the battery 24, an amplifier 25 and an indicating device 26. The wire 22 connects the tube 16 to one pole of the source and the amplifier is connected between the other pole of the source and the metal chamber 11. The output of the amplifier is connected to the indicating device 26.

In operation a stream of scavenging gas is passed into the chamber through the bore 14 and out through the ports 15, from which it may either be allowed to escape to atmosphere or may be collected in a suitable collecting container or vessel. The stream of gas and/ or vapour mixture to be detected passes from the capillary tube 16 into the hollow cylinder 18 which surrounds the electrode 2t and defines the sensing volume. It will be observed that the scavenging gas fiows through the chamber in the opposite direction to the mixture, so that as mixture escapes from the chamber 18 it is immediately picked up by the stream of scavenging gas and carried out of the chamber; thus the ionization characteristics of the gas in the chamber are not altered due to contamination by the mixture.

Where the jacketing cylinder 18 is of very small diameter the apparatus may easily be overloaded and to avoid this the cylinder may, if desired, be made larger. In a modified construction the member 19 is omitted and a cup of larger diameter is formed in the plug 13, the end of the capillary tube being at the bottom of the cup and having its inner and outer diameters rounded. The sensing volume is then at the bottom end of the chamber.

FIGURE 2 shows a modification incorporating a different form of electrode. As shown in that figure the plug 17 is bored out at its end to form a hollow cylinder 18 as before, but the'wire 19 with its metal bead 20 is replaced by a porous metal plug electrode 23 which is inserted in the hollow cylinder 18 and pushed slightly below the upper edge thereof. The mixture issuing from the tube 16 must pass through the plug 23 and the film of gas adjacent the upper surface of the plug 23 provides the sensing volume, which is shielded from the flow of scavenging gas by the edge of the cylinder 18, which projects above the said upper surface and thus forms a rim around it.

The detector described above has been found to perform in a manner closely'similar-to that described in the earlier application. Its sensitivity is of the same order, and is little affected by temperature and pressure. It is however somewhat more sensitive than the previously described detector to the flow rate of the carrier gas. Its sensitivity may be controlled over a wide range by altering the applied potential. For linear operation a series resistance is necessary. The response to substances with ionization potentials less than the excitation potential of the rare gas carrier is closely similar to that of the largevolume version. The sensitivity to substances with ionization potentials greater than the excitationpotential of the rare gas carrier is greater than that of the large volume version, and this fact can find practical use in the analysis of the small group of substances, including the permanent gases, Water vapour and methane, to which the large volume version is relatively insensitive.

While the presently preferred constructions have been described in detail herein, it will of course be understood that many modifications of the invention are possible. Thus, for example, with careful arrangements to ensure a suitable distribution of the flow of scavenging gas through the chamber it becomes possible to dispense with the jacketing cylinder, especially when employing the porous plug type of collecting electrode. When a jacketing cylinder is used, moreover, clearly other materials besides those specified may be employed due regard being paid to their insulating and dielectric properties.

In order to ensure that the scavenging gas does not issue from the bore 14 as a jet which impinges directly on the sensing volume a suitable baffle, such as a piece of wire gauze, may be placed in its path.

I claim:

1. Apparatus for detecting a low concentration of gaseous or vapourized substance comprising a metal cham-' ber, an insulating support projecting into said chamber,

an entry through said support for a mixture of a' rare gas with a substance to 'be detected, an electrode mounted at the inner end of said support close to said entry, said electrode being very small in relation to the volume of said chamber, an inlet in said chamber for a scavenging gas, outlet openings for said scavenging gas spaced from said inlet, a source of ionizing radiation within said chamber, means to apply an accelerating potential to said electrode, and means to measure the ionization level of said gas.

2. Apparatus as claimed in claim 1, used in conjunction with a capillary gas chromatography column, in which the capillary column is constituted by a metal tube and the end thereof passes through said entry into said chamher and constitutes said electrode, the inner and outer diameters of the said endbeing rounded.

3. Apparatus as claimed in claim 1 in which said electrode is a bead formed at the end of -a short length of small diameter metal Wire attached to said end of said column.

4. Apparatus as claimed in claim 1 in which the closed end of said chamber is formed with said entry for said scavenging gas and said outlet openings are formed in said chamber adjacent said insulating support.

5. Apparatus as claimed in claim 4 comprising shielding means to shield the sensing volume in the immediate vicinity of said electrode from the flow of scavenging gas. 6. Apparatus as claimed in claim 5 in Which the shielding means is constituted by a hollow cylinder formed at the end of said support, said elect-rode being inside said cylinder.

7. Apparatus as claimed in claim 6 in which said electrode is constituted by a porous metal plug within said cylinder, the end of said cylinder forming a rim around said porous plug to shield it from the flow of scavenging gas.

References Cited in the file of this patent UNITED STATES PATENTS Bernstein et a1 Dec. 16, 1952 Barnothy Dec. 15, 1959 OTHER REFERENCES Ionization of Alpha Particles in Mixtures of Gases, by

Jesse et al., fromPhysical Review, vol. 100, No. 6, Decem 'April 1956, pages 1958 to 1964. 

1. APPARATUS FOR DETECTING A LOW CONCENTRATION OF GASEOUS OR VAPOURIZED SUBSANCE COMPRISING A METAL CHAMBER, AN INSULATING SUPPORT PROJECTING INTO SAID CHAMBER, AN ENTRY THROUGH SAID SUPPORT FOR A MIXTURE OF A RARE GAS WITH A SUBSTANCE TO BE DETECTED, ANELECTRODE MOUNTED AT THE INNER END OF SAID SUPPORT CLOSE TO SAID ENTRY, SAID ELECTRODE BEING VERY SMALL IN RELATION TO THE VOLUME OF SAID CHAMBER, AN INLET IN SAID CHAMBER FOR A SCAVENGING GAS, OUTLET OPENINGS FOR SAID SCAVENGING GAS SPACED FROM SAID INLET, A SOURCE OF IONIZING RADIATION WITHIN SAID CHAMBER, MEANS APPLY AN ACCELERATING POTENTIAL TO SAID ELECTRODE, AND MEANS TO MEASURE THE IONIZATGION LEVEL OF SAID GAS. 